1. Field of the Invention
The present invention generally relates to magnetic resonance imaging (“MR” or “MRI”) and, more particularly, to devices and methods for selectively enabling transmit and transmit/receive RF coil elements for use with an MRI host device.
2. Description of the Background
MRI systems, and the coils for use therewith, have changed dramatically since the inception of the science in the 1970's. As MR scanners become more powerful and have stronger magnetic fields, the RF coils used to transmit RF signals to the patient and receive a response therefrom have become more precise and better able to achieve a variety of different scan modes. In the use of RF coils, there is a continual tension between transmission power, reception fidelity and a wide range of electromechanical problems (e.g., image artifacts and heightened patient specific absorption rate (SAR) levels).
Initially, MRI systems used large, whole body coils to image subjects, such as human patients. The whole body receive coils of these systems had the advantage that sensitivity was, to a first approximation, substantially constant over the entire region being imaged. While this uniformity in sensitivity was not strictly characteristic of such whole body receive coils, the sensitivity was substantially constant to a degree that most reconstruction techniques assumed a constant coil sensitivity. Due to their large volume, however, whole body receive coils suffer from a relative insensitivity to individual spins.
For certain applications, a local coil (either a surface coil or volume coil which fit more closely to a patient's body part to be imaged) may be preferable to a whole body receive coil in MRI systems. An example of a surface receive coil is described in U.S. Pat. No. 4,793,356 to Misic et al. Surface coils can be made much smaller in geometry than whole body receive coils, and for medical diagnostic use they can be applied near, on or inside the body of a patient. This is especially important where attention is directed to imaging a small region within the patient rather than an entire anatomical cross-section. The use of a surface coil in MRI systems also reduces the noise contribution from electrical losses in the body in comparison to a corresponding whole body receive coil, while maximizing the desired signal. MRI systems thus typically use small local coils (surface or volume) for localized high resolution imaging.
A disadvantage of local coils, especially local surface coils, however, is their limited reception area or field of view (FOV). For example, a single surface coil can only effectively image a region of a subject having lateral dimensions comparable to the surface coil diameter. Therefore, surface coils necessarily restrict the field of view, and inevitably lead to a tradeoff between the resolution of the gathered data (and the resulting image) and the field of view. Generally, large surface coils generate more noise due to their exposure to greater patient sample losses and therefore have a larger noise component relative to the signal, while smaller coils have lower noise but in turn restrict the field of view to a smaller region.
One popular technique that was developed to extend the field of view limitation in local coils (in this case local surface coils) is described in U.S. Pat. No. 4,825,162 entitled “Nuclear Magnetic Resonance (NMR) Imaging with Multiple Surface Coils” and is generally referred to as “phased array.” The '162 patent describes a set of surface coils arrayed with overlapping fields of view. Each of the local surface coils is positioned to have substantially no interaction with any adjacent surface coils (or, more appropriately, the interaction of all of the coils on each other sum to zero). A different response signal is received at each different surface coil from an associated portion of the sample that was enclosed within an imaging volume defined by the array. Each different response signal is used to construct a different one of a like plurality of different images of the sample. The different images are then combined to produce a single composite image of the sample.
In much the same way, the transmission of RF power to a patient's body has caused similar problems in the art. Typically, during a scan procedure, the operator intends to image only a small subpart of the total volume of a patient's body that resides within the bore of the MR scanner. For example, although virtually all of the patient's body may be located within the scanner bore, the operator may only be interested in receiving image data from the patient's left leg. This “intended” portion of the patient (or other object being scanned) is generally referred to as the region of interest “ROI” of the patient.
Any RF power that is transmitted to an area of the patient other than the intended region of interest is wasted (at least) and may cause problems for the patient or the resulting image of the patient. For example, if the left leg is the region of interest, but RF power is applied to the right leg as well as the left leg (e.g., using a whole body coil as the transmitter), the receiver may pick up “artifacts” reflecting off of the right leg. The artifacts may appear similar to “ghost” lines on a television. Moreover, although MRI procedures are extremely safe, it may not always be preferred to send RF power into non-ROI portions of a patient (increased SAR).
To address the foregoing issues, in a similar fashion as with the receive coils discussed above, transmit coils (or transmit coil elements) have also gone through a progression of change through the years. In the early stages of MRI, large, whole body coils were used to transmit RF power to patients, and these whole body coils sent RF power to all portions of the patient within the MR scanner bore. Later, smaller “local” transmit coils were used to transmit RF only to a specific region of the patient within the scanner. Finally, coils capable of selectively switching between transmit and receive modes (T/R coils) were used to address these concerns. Each of these methods improved the artifact and SAR issues, but additional improvement is continually sought.
As phased array receive coils gain popularity, it has been noted that not all of the coil elements that make up the array are necessary for each portion of a scan. For example, as a succession of axial image slices is taken down the longitudinal axis of an RF coil, different coil elements are utilized. Devices and methods to allow for the selection of only some of the plurality of coil elements in an array coil system at different times during a scan have been suggested, but no acceptable solution to this problem has heretofore been provided.
Specifically, in the past, systems and devices have been proposed which dramatically alter the hardware and functionality of the host MR system (i.e., the MR scanner or suite) to control delivery of RF power and the temporal selection of coil elements. For example, non-U.S. patent application number WO2005043182 entitled “B1 Field Control in Magnetic Resonance Imaging” includes one such prior system in which the entire coil interface of the scanner has been redesigned. These prior systems are not useful across a wide variety of existing MR scanners and instead require a specifically-designed scanner for operation. Embodiments of the present invention provide a coil-based device that achieves an intelligent transmit-mode without the need to redesign the coil interface of the MR scanner.
Some companion concepts have recently been introduced into the MR arts. For example, U.S. Pat. No. 6,223,065 to Misic entitled “Automatic Coil Element Selection in Large MRI Coil Arrays” (hereafter “the '065 patent”) includes sensors and logic that detect and alert an intelligent RF coil as to the position of the coil within the bore and as to the identification of the slice being imaged at any given time. It has been contemplated that such sensors/logic can be used to select various receive coil elements so that only elements that are needed to image a specific portion of the current region of interest are activated at any one time. General concepts as introduced above may be incorporated into various embodiments of the present invention.